This invention relates to thermoplastic foam articles, such as drinking cups. In particular, it relates to a novel container structure having improved thermal insulation properties.
Present use of thermoplastic foam cups accounts for many billions of units per year. Foamed polystyrene has been widely employed for hot and cold beverage containers as well as a variety of other uses. Small hand-held tumblers or cups are inexpensive, disposable items commonly supplied as a stack of nestable containers. One product which has found widespread commercial acceptance is the bead-formed polystyrene cup, which is made by introducing a quantity of foamable resin beads containing a thermally activated blowing agent and injecting high temperature steam to cause expansion of the beads inside a cup-shaped mold. Another commercial foam product is fabricated from thermoplastic foam sheet.
In U.S. Pat. No. 3,260,781 (Lux et al) a thermoforming method is disclosed for making a hot cup from thermoplastic foam sheet utilizing plug assist and vacuum molding techniques in combination with compression molding to obtain a cup having low heat distortion. In copending U.S. patent application Ser. No. 942,836, an improved thermoforming technique is disclosed for deep drawing low density thermoplastic sheet with a time-delayed plug-assisted forming step which results in a unique cup structure. Heat retention ability is an important criteria for successful foam hot cups for use with hot liquids. A successful foam hot cup when filled with hot aqueous liquid at the time of use, will maintain that liquid hot for a sufficient period of time. Heat retention is measured by filling a cup with boiling water (100.degree. C.) and measuring the decrease in water temperature versus time. Improving heat retention ability results in maintaining a hot liquid in a cup at a higher temperature at any given time after initial filling.